Writing to low 8/16 bits of register creates false dependency
Generate 32-bit operations when possible.

Change-Id: I8eb6c1c43a66424eec6baa91a660bceb6b80d1d3
Reviewed-on: https://go-review.googlesource.com/19506Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>